1. Field of the Invention
The present invention relates to signal transmission and receiving methods, and more particularly to modulation, demodulation and decoding methods optimized for integrated circuit (IC) implementation.
2. Description of Related Art
Signal modulation is a common data transmission method. Low frequency information signals are transmitted through high frequency waves (called the carrier) by modulating different properties of the carrier such as amplitude modulation (AM), frequency modulation (FM), or pulse phase modulation (PPM). These modulation procedures result in narrow bandwidth waves (called the xe2x80x9cchannelxe2x80x9d) that can be transmitted with minimum interference from signals in other channels. To retrieve the data, a receiver generally has a mechanism to separate the carrier signals in the carrier channel, and another mechanism to extract the information signals from the carrier. Conventional demodulators usually use tuned amplifiers or band-pass filters to receive the carrier signal, and use low pass filters to extract the information. These filters often contain networks of capacitors, resistors, and inductors. For integrated circuit implementation, the design methodology is different from that of discreet circuits. IC technologies are optimized to build switching transistors; millions of transistors can be manufactured in a small area with high yield and excellent uniformity. On the other hand, IC technologies are less effective in building passive components; resistors and inductors in IC occupy large area with relatively poor uniformity. It is therefore a common practice for IC designers to avoid using passive components whenever it is possible to use transistors. One well-known example is the xe2x80x9cswitching capacitorxe2x80x9d (SC) technique that replaces resistors and inductors by switching transistors and capacitors. Filters build by the SC technique is proven to be highly successful for IC implementations, but SC technique is only applicable for low frequency operations. Another approach is to use a phase-locked loop (PLL) to synchronize internal clock signals with the carrier signals; demodulation mechanisms are then executed by switching circuits controlled by internal clock signals. PLL is proven to be highly successful for many IC implementations. Its major problem is stability. As a sensitive analog feedback circuit, stability problems such as phase gitter and noise sensitivity are inevitable for PLL circuits. There are many other inventions developed to improve modulation or demodulation circuits for IC implementation. In U.S. Pat. No. 4,501,002 Auchterlonie described a demodulator for quadriphase shift keying (QPSK) signals. This invention is useful only for digital signals carried in QPSK format; it is not useful for signals in other formats, and it does not have the accuracy to decode analog signals. In U.S. Pat. No. 5,477,199 Montreuil described another system that is able to demodulate digital data in both vestigial side band and offset keyed quadriphase signals. Again, the invention is useful only for specially formatted signals, and it is only for digital data decoding. Components used in Montreuil patent are also not optimized for IC implementation. It is highly desirable to be able to manufacture general purpose signal transmission and receiving circuitry optimized for IC technologies. It is also desirable to avoid using passive components or sensitive circuits such as PLL.
The primary objective of this invention is, therefore, to provide practical data transmission and receiving circuits optimized for integrated circuit implementation. The other primary objective of this invention is to improve reliability and stability of data transmission circuits. Another objective of the present invention is to provide practical methods to reduce transmission errors caused by non-ideal conditions. Another objective of the present invention is to provide the capability to transmit modulated signals that are not detectable with conventional demodulation methods. Another objective of the present invention is to maximize data transmission rate by demodulating both AM and FM signals simultaneously. It is a further objective of the present invention to provide a method to transmit and receive AM, FM, and digital data from the same carrier signal.
These and other objectives are accomplished by novel asynchronous demodulation methods. The demodulation methods extract information from carrier signals by switching transistors controlled by internal control signals. Using multiple control signals with different phases, the demodulation methods of the present invention does not need to synchronize the internal control signals with the input carrier signals. It is therefore possible to avoid using sensitive analog circuits such as PLL or mixers. Demodulators of the present invention comprise mostly switching transistors; there are no passive resistors and inductors. It is therefore ready to take full advantage of existing IC technologies to achieve highest performance using smallest area. Reliability and stability are improved by avoiding sensitive linear circuits or unstable feedback circuits. Circuit stability is further improved by a mechanism in the internal reference control signals that significantly improves circuit tolerance on non-ideal operation conditions. The demodulation methods of the present invention have a special property called xe2x80x9cquadrant independencexe2x80x9d. When the phases of individual cycles of the carrier signal are shifted by an integral of 90 degrees, the demodulation results do not change. Utilizing this special property, it is possible to transmit signals in a way that current art demodulators won""t be able to detect. It is also possible to improve data transmission rate by transmitting and receiving AM, FM, and digital signals simultaneously.
While the novel features of the invention are set forth with particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.